Its bad practice to reuse any of those gaskets. Leaks at any of those places will effect how your car runs. Want to reuse exhaust gaskets? Reuse catback gaskets. Just an opinion and you know what opinions are full of.

If I have to reuse one I clean it, and spray on a heavy coat of UltraCopper high tack sealant, on both sides. On very thick coat, let it dry completly. I have had good results with the ultra copper stuff.